It has been shown that specific force (Po/CSA) of identified types of atrophic muscle fibers of sedentary and immobilized elderly subjects (EL) is lower than that of young healthy (YO) subjects and that a determinant of such loss could be a lower myosin concentration in muscle fibers. It is unclear if this phenomena occurs in disuse muscle atrophy in YO healthy subjects and in normally active EL subjects. We performed two studies in which torque and muscle volume of the quadriceps muscle were determined in vivo; cross sectional area (CSA), Po/CSA and myosin concentration of isolated muscle fibers from biopsy samples of vastus lateralis were determined in vitro. Study 1: 4 YO subjects were subjected to 3 weeks unilateral lower limb suspension (ULLS); isometric torque and muscle volume by MRI were lower (19% and 12% respectively) post-ULLS compared to pre-ULLS and increased (12% and 17%) post-3 weeks recovery. CSA and Po/CSA of muscle fibers were lower (15% and 13%) post-ULLS compared to pre-ULLS. Study 2: specific force and volume by MRI were lower (18% and 27%) in normally active EL subjects compared to YO controls; muscle fibers from EL subjects had a 14% in CSA and a 25% of Po/CSA decrease compared to YO subjects. The myosin concentration analysis was unchanged in EL subjects. This analysis in ULLS subjects is still ongoing. Our data indicate that, in moderate atrophy, other mechanism such as post-translational modification may represent the determinant of force loss.